What do Chromatograms tell us? Base line goes up and shows a “hump” after each analysis..8 Aug 2014
Chromatograms are like fingerprints. If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.
When using GC sometimes the baseline increases with every analysis. Also sometimes a huge “hump” elutes after all the components of interest have eluted, see fig.1. The “hump” typically shows as a broad peak, indicating “something” is eluting form the column. Normally the base line should be stable after the elution of the peak marked “X”.
Fig.1 Ghost peaks appearing every analysis
Typical causes for such “humps” are:
Built up of later eluting peaks in the column. If there are heavy components in the sample, they will elute eventually. This will result in unstable base line and also in the “hump” from figure 1.
- Heat the column to a higher temperature each analysis, so the heavier components will elute.
- If the column cannot be heated at higher temperature, consider to use a flow – or pressure program. This will also make the late eluting compounds elute faster.
- Use a back flush system. You can back-flush the whole column after elution of the last peak of interest; You can also use a pre-column of the same phase and only back-flush the pre-column. Back-flush systems are used a lot in process analyzers. They will offer the most reliable chromatography. Challenge is, that valves or Deans switching has to be used, which does require a little more understanding of the separation process.
- One can also choose to do more sample prep and remove heavies before the analysis is done.If the “hump” or ghost-peak is not coming from the sample, it may be a contaminant somewhere in the system, that is also accumulating. Check this by doing a blank run (no injection) first; Then also do a solvent injection and see if the “hump” is increasing. GC split/splitless systems often get contaminated at the split-vent line. Because this location is cold, often heavy components can accumulate. See also Blog: Contamination of Injection System Split-Vent-Lines: A Maintenance item not to Underestimate
That split-vent line need to be cleaned regularly, as well as the charcoal trap that should be connected downstream your split-vent line. Be aware of that. Many of us do not realize this, but it can impact the base line significantly. Lastly, also check your detector temperature.. If it’s too low, peaks will also broaden..
- Using a solvent, especially in splitless mode, sometimes extracts these contaminants and they get into the column. Ideally this should not happen as there should be a positive flow outside the injection port. Practically, we do see sometimes, that such contaminants can make it back into the system and they show up as ghost / broadened peaks.